Force a full GC after importing a project

During a project import, it's possible that new branches are created by
the importer to handle pull requests that have been created from forked
projects, which would increment the `pushes_since_gc` value via
`HousekeepingService.increment!` before a full garbage collection gets
to run. This causes HousekeepingService to skip the full `git gc` and
move to the incremental repack mode. To ensure that a garbage collection
is run to pack refs and objects, explicitly execute the task.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/59477

app/services/projects/after_import_service.rb

@@ -9,7 +9,7 @@ module Projects
     end
 
     def execute
-      Projects::HousekeepingService.new(@project).execute do
+      Projects::HousekeepingService.new(@project, :gc).execute do
         repository.delete_all_refs_except(RESERVED_REF_PREFIXES)
       end
     rescue Projects::HousekeepingService::LeaseTaken => e

app/services/projects/housekeeping_service.rb

@@ -18,8 +18,9 @@ module Projects
       end
     end
 
-    def initialize(project)
+    def initialize(project, task = nil)
       @project = project
+      @task = task
     end
 
     def execute
@@ -69,6 +70,8 @@ module Projects
     end
 
     def task
+      return @task if @task
+
       if pushes_since_gc % gc_period == 0
         :gc
       elsif pushes_since_gc % full_repack_period == 0

changelogs/unreleased/sh-force-gc-after-import.yml

@@ -0,0 +1,5 @@
+---
+title: Force a full GC after importing a project
+merge_request: 26803
+author:
+type: performance

spec/services/projects/after_import_service_spec.rb

@@ -13,7 +13,7 @@ describe Projects::AfterImportService do
   describe '#execute' do
     before do
       allow(Projects::HousekeepingService)
-        .to receive(:new).with(project).and_return(housekeeping_service)
+        .to receive(:new).with(project, :gc).and_return(housekeeping_service)
 
       allow(housekeeping_service)
         .to receive(:execute).and_yield

spec/services/projects/housekeeping_service_spec.rb

@@ -88,6 +88,19 @@ describe Projects::HousekeepingService do
         expect(project.pushes_since_gc).to eq(1)
       end
     end
+
+    it 'runs the task specifically requested' do
+      housekeeping = described_class.new(project, :gc)
+
+      allow(housekeeping).to receive(:try_obtain_lease).and_return(:gc_uuid)
+      allow(housekeeping).to receive(:lease_key).and_return(:gc_lease_key)
+
+      expect(GitGarbageCollectWorker).to receive(:perform_async).with(project.id, :gc, :gc_lease_key, :gc_uuid).twice
+
+      2.times do
+        housekeeping.execute
+      end
+    end
   end
 
   describe '#needed?' do